1. Field of the Invention
This invention relates in general to hardfacing, and in particular to hardfacing applied to bit legs and cones of a rolling cone earth-boring bit.
2. Background of the Art
In drilling bore holes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters or cones are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface, or the bit is rotated by downhole motors or turbines. The cutters or cones mounted on the bit roll and slide upon the bottom of the bore hole as the bit is rotated, thereby engaging and disengaging the formation material to be removed. The rolling cutters are provided with cutting elements or teeth that are forced to penetrate and gouge the bottom of the borehole by weight of the drillstring. The cuttings from the bottom sidewalls of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow drillstring. A rounded end of the bit leg that corresponds with the cutter is commonly referred to as a shirttail.
Before the cuttings are washed away, the cuttings slide over portions of the drill bit while the bit is rotating. The cuttings are abrasive and can cause wear on the surfaces of the drill bit, which can eventually lead to failure. When faced with wear problems, especially in the art of the cutting elements on the cutters, it has been common in the arts since at least the 1930s to provide a layer of wear-resistance metallurgical material called “hardfacing” over those portions of the teeth exposed to the most severe wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast, or macrocrystalline tungsten carbide, dispersed in a metal matrix. Such hardfacing materials are applied by welding a metallic matrix to the surface to be hardfaced.
A typical technique for applying hardfacing to the cutting elements by oxyacetylene or atomic hydrogen welding. A welding rod or stick is typically formed as a tube of mild steel sheet enclosing a filler that mainly comprises carbide particles. The filler may also include deoxidizer for the steel, flux and a resin binder. The hardfacing is applied by melting an end of the rod on the face of the tooth. The steel tube melts as it is welded to the steel tooth and provides the matrix for the carbide particles. The deoxidizer alloys with the mild steel of the tube.
Increased wear resistance of hardfacing is desirable to lengthen distance the drill bit can drill before dulling. Wear resistance also allows the drill bit to cut more efficiently, and therefore faster, to such depths. Accordingly, the benefits from increased wear resistance reduce the costs drilling in both parts and time.